1. Field of the Invention
The present invention relates to an electric power steering apparatus in which a three-phase brushless motor is used to generate a steering assist force. In particular, the present invention relates to the configuration of the control circuit thereof.
2. Description of the Related Art
An electric power steering apparatus is an apparatus in which a motor is used to generate a steering assist force in accordance with a steering torque caused in a steering shaft. The motor is generally a three-phase brushless motor. The motor is mainly controlled by a CPU and an electronic circuit. When the normal operation of the electric power steering apparatus cannot be guaranteed due to the malfunction of the CPU or the failure of the electronic circuit in the worst case, the motor must be securely stopped. To realize this, a circuit configuration has been used in which a feeding current path to the motor includes a relay contact so that the contact is opened when the CPU or electronic circuit has abnormality to securely stop the feeding to the motor (see Japanese Published Unexamined Patent Application No. 2006-182166 for example).
The above contact must be closed when the electric power steering apparatus operates normally and must be conductive with a favorable electrical conductivity. However, when the surface of the contact is oxidized for example to have thereon an insulating film or when dusts existing at the periphery of the contact or fine abrasion powder of the relay itself for example are attached to the surface of the contact, these foreign matters cause an increase in the contact resistance of the contact, thus reducing the electrical conductivity. The reduced electrical conductivity causes an adverse influence on the motor control. Thus, a function is required to check whether the contact has a favorable electrical conductivity or not when the electric power steering apparatus is started. Another function is also required, in addition to the mere check function, to solve or reduce a minor or temporary reduction in the electrical conductivity to recover a favorable electrical conductivity.
It has been considered effective to remove foreign matters as described above by chattering the contact (see Japanese Published Unexamined Patent Application No. S63-237319 for example). As a part of an initial operation prior to a general motor control, a processing has been suggested in recent years according to which a predetermined voltage small enough not to operate a motor is applied to the motor to remove foreign matters (see Japanese Published Unexamined Patent Application No. 2006-211825 for example).
In the conventional techniques as described above, however, the chattering and voltage application may not remove foreign matters in some cases. Due to the existence of such foreign matters, a mere voltage application finds it difficult to provide a secure conduction check.